- Docker Compose – depends_on, Healthchecks and Startup Order
`depends_on` waits for the container to start, not for the service inside it to be ready — so the app races MySQL and dies on a connection refused that looks like a database bug. Writing a healthcheck, `condition: service_healthy`, `up --wait`, and why the app should still retry anyway.
- Hasura – Event Triggers and Scheduled Triggers
Running code when data changes, without polling. Event triggers fire a webhook on insert, update or delete and retry it with a backoff; scheduled triggers do the same on a cron or at a one-off future time. Delivery guarantees, why your handler must be idempotent, and how to inspect and replay a failed event. Shown on a real bookings table — and with the awkward v3 news stated plainly: event triggers are still work in progress and cron triggers are not supported at all.
- Spring Boot – Configuring the Security Filter Chain
The modern lambda DSL — WebSecurityCustomizer is gone and so is WebSecurityConfigurerAdapter. Ordering rules so a permitAll does not accidentally open an admin endpoint, disabling CSRF only where it is genuinely safe to, and making CORS and security agree instead of fighting.
- React – Refs
A ref is a value a component remembers that does not trigger a render when it changes. Reaching a real DOM node to focus, measure or scroll it, holding a timer id between renders, why reading or writing a ref during render is a bug, and how ref differs from state in one table.
- Python – Decorators
Decorators, built up from the one fact that makes them possible: functions are objects. Writing one, passing arguments through it, why `functools.wraps` is not optional, and the built-in decorators you already use.